Mixer shower question (water delay)

[ss]

Image link below:

The mixer taps on shower are having a delay (approx 20 seconds) when the water is turned on then runs ok so suspect either the filters are blocked or the cartridge is at fault. (already checked the shower head/spray) In any case I need to take the taps off to investigate.(Mira Coda shower)

Question: I need to undo the 2 nuts as indicated, can I just undo them or do I need to support the tap assembly in some way to counter the torque when spannering the nut?

If the filters are not blocked then what am I looking for with regard to the cartridge (have only ever had old washer type taps before)

[John Rumm]

The nuts will typically be captive on the shower valve, but freely rotating - so no need to support the valve since it will not see any torque. Chances are the protruding thread left sticking out of the wall will be the 3/4" BSP end of a 1/2" BSP to 3/4" BSP "cranked" coupler (with 1/2" BSP female sockets set flush with the finished surface of the wall - all concealed by the chromed caps).

If its a hard water area, some descaling might do it. You should be able to get replacement cartridges if needs be.